It is essential in monitoring pH that the sensor, usually a glass electrode, does not become so fouled as to prevent the sensor surface being in direct contact with the sample. With a glass electrode, or other potentiometric sensor, a substantial area of the sensor surface must be free of fouling.
It is found that fouling of a reference electrode junction, unless exceptionally heavy, rarely leads to a significant problem. However, even mild fouling of a glass electrode can quickly result in a sluggish response to changing sample pH and eventually to a complete lack of response. Such behaviour can occur relatively rapidly, often within a few hours. It is therefore not possible to deal with the problem in any sensible cleaning/maintenance schedule.
It will be appreciated that these observations apply to a wide variety of electrometric measurements that rely on direct contact between a sensor surface and a sample. Related problems may also arise with other forms of sensor surface such as a window provided for an optical sensing measurement.
Manufacturers of sensors have hitherto offered a variety of so-called electrode cleaners. This term is generally a misnomer as the use of many of these devices is aimed at preventing or reducing fouling. These "cleaners" may be roughly classified into three main groups: ultrasonic, mechanical and chemical.
Ultrasonic devices are usually excellent in preventing crystallization from a liquid sample onto the electrode surface (for example calcium sulphate in water treatment plant); in maintaining light oils and solvents in aqueous suspension and in reducing the slow deposition of fine flocculants. The basis of the technique is of high frequency vibrations generated from an emitter, causing cavitation at the electrode surface preventing or removing depositions. The disadvantage of the ultrasonic approach is principally that of cost.
A typical known mechanical cleaner takes the form of a brush moving across or around the sensor surface driven externally, either electrically or with compressed air. The individual bristles of the brush are commonly of plastic and are stiff. Generally they are ineffective against oil or grease contaminants or where crystallization may occur and are not recommended where the sample may contain abrasive matter which would damage the sensor surface. An alternative approach is suggested in DE 1217656 which shows a frame rotated about the bulb of a pH electrode, the frame carrying stretched elastic strips which rub over the bulb surface. An expensive drive arrangement is required to rotate the frame.
With reference to U.S. Pat. No. 4,285,792, there has been proposed a paddle wheel driven by the flow of sample liquid and carrying bristle cleaning brushes which sweep over the surface of a pH electrode. Whilst the expense of a separate drive arrangement is in this way avoided, the rotatable paddle wheel represents a mechanical complexity in the measurement cell and operation can only be guaranteed at relatively high flow velocities. Further alternative proposals for a mechanical cleaner utilise the motion of plastic (PTFE) balls or a foamed plastic body (see DE 3405234) captured within a suitable housing around the sensor surface and powered by the sample flow physically to prevent accumulation of coating materials on the sensor surface. Significant flow rates are recommended to maintain sufficient motion of the plastic balls and the arrangement of course requires a chamber surrounding the sensor, to contain the balls or other body.
Unlike mechanical and ultrasonic "cleaners" which do not interfere with the measurement and maybe continuously operated, chemical cleaners have the disadvantage of requiring system isolation from the sample stream during the cleaning cycle. This usually entails the provision of bypass lines, valves and perhaps a drain to waste. They comprise a jet or ring of jets directed at the sensor surface. On activation, reagent is under electrical power pumped via the jets onto the sensor surface.